Alan Stern 写道: > On Tue, 9 Jan 2007, xiaoxin liu wrote: > > >> Hi, Alan >> Thanks for your reply. >> I still have some questions as follows: >> > > >>> Your driver must keep the endpoint queue full at all times. In practice >>> this means that you must keep several URBs queued, and each time one of >>> them completes your completion handler routine must submit another URB. >>> >>> >> You mean that I should resubmit a URB in completion handler in my >> driver. >> > > Yes, that's what I meant. > > >> However, >> Currently I submit isochronous transfer request from user application >> through ioctl(), not in a kernel usb device >> driver. In fact, the completion handler routine is defaultly set as >> async_completed(). That is to say, >> I can't modify the completion handler to resubmit a URB. >> What suggestions for this situation?? >> > > Do you mean that your driver runs entirely in userspace? > Yes, that is right. In fact, it is a part of libusb, which is an open source project in sourceforge. > You can still keep the endpoint queue full, but the procedure isn't quite > as reliable. All you have to do is submit enough URBs in advance so that > the queue never gets empty, even given the normal overhead and latency of > running user programs. So for example, let's assume that your system has > 100-ms latency for task-switching to your program. Then you would need to > keep at least 100 ms worth of data in the endpoint queue at all times. > It doesn't all have to be in one URB; in fact you're better off splitting > it among several URBs. For instance, you might submit 5 URBs, each of > which contains 25 ms of data. > you mean while doing isochronous transfer in user space I should submit servel URBs together in the first time. Then resubmit urb in each urb's completion handler. Am I right? > And like I said before, every time an URB completes you must submit a new > one. You don't have to resubmit in the completion handler itself; the > handler can set a variable telling the main program how many URBs to > resubmit. > > Alan Stern > > Xiaoxin
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